The diving tank is a crafting material in How to Survive: Storm Warning.
Diving tanks are used in the crafting of handmade firearms as well as the flamethrower. Alternatively to a manual compressor, when a diving tank is used in the creation of handmade gun, it will create guns with higher fire rates, like the sten gun.
The first diving tank is acquired when Kovac gives the quest (enter name of quest); the second in his quest chain. After completing the quest, returning the fast shotgun to Kovac and continuing the game, diving tanks are found strewn about the archipelago.
Diving tank + harpoon grip = unfinished fast weapon (used in fast shotgun, sten gun, (insert machine gun link).
Diving tank + unfinished gas weapon part 1 = Unfinished gas weapon part 2 (flamethrower crafting)